Abstract
Seasonal variations of liquid and ice water paths (LWP and IWP) in nonprecipitating clouds over oceans are estimated for 4 months by combining the International Satellite Cloud Climatology Project (ISCCP) and Special Sensor Microwave/Imager (SSM/I) data. The ISCCP data are used to separate clear/cloudy skies and warm/cold clouds and to determine cloud optical thickness, cloud-top temperature, and sea surface temperature. SSM/I data are used to separate precipitating and nonprecipitating clouds and to determine LWP. About 93% of all clouds are nonprecipitating clouds, and about half of nonprecipitating clouds are warm (cloud-top temperature > 0°C). The average LWP for warm nonprecipitating clouds is about 6 mg cm−2. The values of total water path obtained from the ISCCP values of optical thickness for cold nonprecipitating clouds are larger than the LWP values from SSM/I, which the authors explain in terms of IWP. The average IWP for cold nonprecipitating clouds is about 7 mg cm−2, with LWP being about 5 Mg cm−2. Tropical and cold hemisphere clouds have higher IWP values (around 10 mg cm−2) than those in warm hemispheres; where LWP values for warm nonprecipitating clouds vary little with latitude or season. Ice fractions, IWP/(LWP + IWP), in cold nonprecipitating clouds increase systematically with decreasing cloud-top temperatures, reaching 50% at about −15°C but ranging from about −5° to −10°C in the northern midlatitudes in autumn and the Tropics year-round to about −25°C in the southern midlatitudes in summer. The ratio of IWP to LWP in cold nonprecipitating clouds reaches almost 3 in the northern midlatitudes in autumn and falls as low as 0.6 in the southern midlatitudes in spring-summer. Combining warm and cold nonprecipitating clouds gives a global ratio of IWP to LWP that is about 0.7 over oceans.
